In many establishments such as casinos a large number of key operated cylinder locks are used. Existing cylinder locks typically comprise a rotatable plug or core within a nonrotating cylinder which is secured to the lock body. The rotatable plug is releasably secured against rotation by means of locking pins which are spring loaded to extend across the shear plane between the plug and cylinder. The pins are activated by a coded key which when inserted into the plug aligns ends of the locking pins with the shear plane such that said plug may be rotated to disengage a cam tail or other latch mechanism. Subsequent removal of the key after returning the plug to its latched position releases the pins to secure the plug against further rotation. The disadvantage of these prior art systems is that when a coded key is lost the security of the establishment can only be restored by rekeying all the locks for which it was used. In casinos or other establishments having a large number of poker machines, each fitted with several locks the cost of this is considerable.